The research conducted for this project is directed toward an analysis of conditional stimulus control over visual choice selection by normal and developmentally delayed persons. Emphasis is placed upon understanding principles involved in bringing selection of choice stimuli under conditional control of spoken words and visual samples or standards. Previous research has demonstrated that delayed persons, relatively sophisticated linguistically, can be trained to select one stimulus choice (out of two available choices) in the presence of one spoken word. The results were interpreted as: "If the spoken word is the one from the training condition, select the simulus choice leading to reinforcement in training. If the spoken word is 'different,' select the previously incorrect stimulus." Current research is investigating the accuracy of stimulus choice selection under varying ratios of presenting the untrained or different spoken word relative to the spoken word from the training condition. The pilot data suggests moderately retarded persons will maintain accurate stimulus choice selection when the different or untrained spoken word is presented infrequently relative to the spoken word from training (1:8 ratio of presentations). However, accuracy falls to chance performance if the two words are presented at equal frequency after training on one spoken word.